I have basically the same load: Alsa 124gr FMJ RN, 8.6 grs of 3N38 from an old STI matchmaster with a 4,25" barrel but I'm getting barely an average of 163 PF.
Now I'm having a Brazos 5" barrel with TC2 cuts. Any suggestions on how much to use 3N38? Just by looking at the post above, 8.6 grs might be way too much. Just looking for a ballpark start. Maybe 8.2?